¿Escribir en un archivo .txt?

FILE *f = fopen("file.txt", "w");
if (f == NULL)
{
    printf("Error opening file!\n");
    exit(1);
}

/* print some text */
const char *text = "Write this to the file";
fprintf(f, "Some text: %s\n", text);

/* print integers and floats */
int i = 1;
float pi= 3.1415927;
fprintf(f, "Integer: %d, float: %f\n", i, pi);

/* printing single characters */
char c = 'A';
fprintf(f, "A character: %c\n", c);

fclose(f);

FILE *fp;
char* str = "string";
int x = 10;

fp=fopen("test.txt", "w");
if(fp == NULL)
    exit(-1);
fprintf(fp, "This is a string which is written to a file\n");
fprintf(fp, "The string has %d words and keyword %s\n", x, str);
fclose(fp);